Recent advances in genetic engineering have provided the prerequisite tools to transform plants to contain foreign (often referred to as “heterogeneous or heterologous”) or improved endogenous genes. The introduction of such a gene in a plant can desirably lead to an improvement of an already existing pathway in plant tissues or introduction of a novel pathway to modify desired product levels, increase metabolic efficiency, and/or save on energy cost to the cell. Plants with unique physiological and biochemical traits and characteristics, such as herbicide resistance and insect resistance, have already been produced. The ability to create traits that play an essential role in plant growth and development, crop yield potential and stability, and crop quality and composition are particularly desirable targets for the crop plant improvement.
Normally a plant goes through a development cycle, which includes seed germination, maturation of plant, reproduction, and finally senescence that leads to death of a plant. Several biological processes are common to different stages of plant development. Desired effects such as growth of tissue organ are achieved in nature by fine-tuning of the metabolism of the organism. The final phase of growth is senescence which is a highly regulated, genetically controlled and active process (Thomas H., and Stoddart J. L., Ann. Rev. Plant Physiol (31) 83-111, 1980). Senescence is mostly studied in plant leaves and is regarded as a series of events concerned with cellular disassembly and the mobilization of released material to other plant parts such as seeds, storage organs or developing leaves and flowers (Nooden L. D. In Senescence and Aging in Plants, Academic press, 391-439, 1988). Leaf senescence can be initiated by seed development in certain species of plants. This was demonstrated in soybean by surgically removing flowers or physically restricting pod growth to observe the delay in leaf senescence (Nooden L. D. In Senescence and Aging in Plants, Academic press, 330-368, 1988; Miceli F, Crafts-Brandner S. J., Egli D. B. Crop Sci. (35), 1080-1085, 1995). During senescence, partitioning of resources between vegetative and reproductive development involves a complex interplay of generative and degenerative processes, requiring differential expression of genes.
Differentially expressed genes during senescence are usually referred as “Senescence Associated Genes” or SAGs (Hensel L. L, Grbic V., Baumgarten D. A., Bleecker A. B., The Plant Cell (5) 553-564 1993). All SAG genes may not be functionally related, but they all are involved in similar physiological processes. In the past, senescence studies were directed towards understanding processes to generally enhance knowledge and applying this information relating to senescence in agriculture to enhancing yield and reducing post harvest losses (Hensel L. L, Grbic V., Baumgarten D. A., Bleecker A. B., The Plant Cell (5) 553-564 1993; Gan S., Amasino R. M., Science, (270) 1986-1988, 1995; Gan S., Amasino R. M., Plant Physiol., (113) 313-319, 1997; Guarente L., Ruvkun G., Amasino R. M., Proc. Natl. Acad. Sci. USA (95) 1034-1036, 1998; U.S. Pat. No. 5,689,042; PCT/US00/03494; and PCT/US00/18364, July 2000).
The SAG 13 gene was first described by Lohman et al. in 1994 (Lohman K. M., Gan S., John M. C., Amasino R. M., Physiologia Plantarum (92) 322-328, 1994) and then by Weaver et al. in 1998 (Weaver L. M. Gan S., Quirino B, Amasino R. M., Plant Mol. Biol. 455-469, 1998) as one of the genes associated with senescence. SAG genes by definition are up regulated during age-mediated senescence. SAG 13 was observed to be induced strongly shortly before visible senescence marked by yellowing of green leaves.